Logarithm of europium concentration
by



April 6, 1965 R, R. SODEN ETAL 3,177,156

OPTICAL MASER CRYSTALS Filed Aug. '7. 1961 FIG.

CURVE 40 RELATIVE EMISSION INTENSITY CURVE 2 l I I LOGARITHM OF EUROPIUMCONCENTRATION R. R. sous/v INVENTORS'LG. v4/v U/TERT ATTORNEV Thisinvention'relates to single crystal tungstate materials exhibitingfluorescent properties andto devices utilizing such crystals.

Recently, considerable interest has developed in a new class of solidstate maser devices'in which the stimulated fre uenc is in the o ticalor near ontical s ectrum inl P cluding the infrared .and ultravioletportions of the electromagnetic spectrum. This spectrum encompasses thewavelength range of from 100 A. to 2x13 A. in principle, these devicesare directly analogous to the microwave maser, and the mechanics oftheir operation are well detailed in the literature, for example asdescribed by A. in. Schawlow and C. H. Townes in.U.S. Patent 2,929,922,issued March 22, 196i). v

Among the more promising forms of optical masers are-those which employa material whose energy level system is characterized by at least threeenergy levels, with the separation of these'levels falling within thedesired operatingfrequency ranges. During operation, there isestablished, at least intermittently, a nonequilibrium electronpopulation distribution in a panel the selected three energy levels. Inparticular, the population of the higher of the selected pairs of energylevels is increased to the point at which it is greater than that of thelower'level. it is cusomary torefer to a'material in such a state ofnonequilibrium as exhibiting a negative: temperature.

United States Pat F I It is characteristic that if there is applied to amaterial in a negative temperature state. a signal of a frequency whichsatisfies ldancks Law with respect to the we energy levels innonequilibrium, the applied signal will stimulate the emission ofradiation in phase with the signal frequency from the material and thesignal will be amplified. in other Words, the active maser material ischosensuch that the two energy levels are separated by an energy equaltoih where his Planclrs constant and fl is equal to the frequency to beamplified. This separation is less than the separation between the topand bottom levels of the selected three-level energy system. V

The negativetemperature state is established by .apply-. ing to thematerial pump energy of a frequency of at least they frequencycorresponding to the separtion between the top and bottom levels of theselected three-level energy'system. The applicationof. sufficient pumpenergy ellects electron transitions from the bottom level to the toplevel and the populations of the bottom and top levels are thereby madeto approach equality. Under these conamass Apr. 6, 1%65 transition to astate other than the ground state such that the single bright emissionline is narrow in width.

Since the pump sources typically utilized in optical masers generallyexhibit an energy output over a broad frequency spectrum, it isdesirable that the paramagnetic ions possess a broad absorption spectrumto facilitate establishment of the negative temperature state. Desirablythe paramagnetic ions also exhibit a relaxation time sufficientlylong sothatthe quantum eiiiciency for fluorescence is close to unity.Otherwise, the magnitude of the pumping frequency would have to begreatly increased in order to maintain a negative temperature statewherein suihcient electrons are available in the higher energy level toamplify the input frequency. To ensure a narrow emission line, theenergy level widths of the pair of spaced energy levels in the negativetemperature state are preferably narrow.

In View of the above-detailed requirements, very few optical masermaterials are lmown to the art. published work on optical masers isdirected to ruby crystals and calcium fluoride crystals containing smallamounts of uranium (H1) and samarium (ll). Ruby crystals, however,suffer the disadvantage of requiring high pumping power to establish anegative temperature.

state. As such, under the usual conditions ruby masers are limited inoperation to producing a pulsed beam of coherent light.

As previously discussed, there should be a correspondence between thesignal to be amplified and the energy level separations of the masermaterial. herefore, it is desirable that new maser materials having arange of energy level separations and fulfilling the abovedetailedrequirements be developed so that a range of signal fre- Where A is anion selected from the group consisting of ulutetiunnlyttrium, lanthanum,and gadolinium, B is a ditions there will be a negative temperatureeither be- 7 tweenthe top and middle'levels or between the middle andbottom levels. Since a competing process known as relaxation tends toreturn the .system'to equilibrium, thereby destroying the negativetemperature state, continuous pump energyis applied to the materialduring the period'of signal amplification. 7

.Among the more promising active maser materials are those whichcomprise a. host crystal containing paramagnetic, ions from whichthestimulated emission occurs. The host crystal of a material meeting theabove described requirements must be capable of accepting theparamagnetic ions in such a way that they are able on excitationtofluoresce with good over-all quantum efficiency with as much of theemitted energy as possible concentrated in trivalent rare earth ionselected from the group consisting of europium, terbium, and ytterbium,and x has a value of 0.06 to 0.45. The subscripts in the above formulasignfy the relative number of gram atoms of the element indicated whichare present, and thus are also proportional to the relative number ofatoms of each element present in th composition. 7

The compositions of the instant invention emit energy of narrow linewidth. For example, the line width of NflojYogElloqVv/Og associated withan emitting wavelength of approximately 6150 A. is in the order of 7 to9 cin at liquid nitrogen temperature. The excited electrons evidence arelaxation time sufilciently long so that the quantum efiiciency forfluorescence is close to unity. Since the ions possess at least threeenergy levels and electron transitions are to other than the groundstate, the establishment of a continuous negative temperature state .is'feasible and the material is capable of ,fiuorescing in a continuousbeam of coherentlight. The invention may be more easily understood byrefen once to the drawing, in which: I I r FIG. 1 is a front elevationalview of an apparatus utilizing the composition of the invention; and

HG. 2,v on coordinates of relative emission intensity and gram atoms performula of trivalent europium ion,

Most

within the scope of the invention.

isasemirlog plot showing the dependency of the-emission intensity on theconcentration of rare earth ionin the" material of the invention.

' Referring more particularly to FIG. 1, there is shown a rod shapedcrystal 1 having the. composition as disclosedherein. Pump energy is.supplied by means of helical lamp 2 encompassing rod 1 and connected toanf containing compositions, 10,000 A. for the ytterhium containingcompositions and 5450 A. for the terhiurn containing compositions of theinvention. Rod 1 during operation is preferably maintained in anatmosphere of liquid nitrogen (at a temperature of approximately 79 K.)so as to more readily attain a negative temperature state. 1

The spectrum of the pump source including ultraviolet light is desirablyWithin the range of 2,000 A. to 4,200 A. Although higher frequencies aresuitable, sources of such frequencies are not generally available. Ithas been found that an ultraviolet light source having a peak of 3660A.is most advantageous forthe present purposes.

Although the expressed range is the range of energy most eifective, itis not necessary to use a source having an output restricted to thisrange. For example, a gaseous discharge flash bulb, although emittingwhite light,nevertheless emits a large amount of energy in the desiredspectrum.

, Device discussionhas been largely in terms of the.

most commonly reported maser design. Although such a device is easilyfabricated, other configurations have been disclosed in the literatureand' may, prove advantageous. All such variations are consideredto beThe etiect of rare earth ion concentration on the emis sion intensity ofthe material of the invention is shown by curve 1 of FIG. 2., Inthisfigure, the, ordinate meas- Although not plotted, measurements indicatethat luteti- Curve 1 of this figure-shows the crystals in which the,paramagnetic ions fall-within the prescribed limits astrivalent ions.

Based on the preceding considerations, a preferred rare earth ioninclusion range is 0.08v to 0.4 gramatoms per formula, with an optimumrange being 0.10- to 0.35 gram atoms per formula.

To obtain curves 1 andZ of FIG. 2, rneasurements were made onvariousmaterials of the present invention with a Gaertner highdispersionspectrometer adapted with an AMINCO photomultiplier. using a IP22 tube.Ten micron slit widths were employed at the entrance and exit to thespectrometer. The systemwas calibrated againsta tungsten filament lamp,whose output was assumed to'have a black-body. dependence, to" giverelative values of brightness of the emitting surface in units of powerper unit wavelength .range. Emission was excited by illuminating asample one inch long by one-half inch wide by one-quarter inch deep witha 3660 A. rich H4 spotlampthrough a Corning 5874 filter. The measurements were made at room temperature and the inten sities arerelative'to 100 for the 6150 A." peak of a comparable sample of Na EuWO- It has been determined that replacing luteti'um, yttrium, lanthanum,and gadolinium with other Groupv 1H ions, for example aluminum, gallium;orscandium, results in a different structure which does not accommodateappreciableqquantities of the fluorescent rare earth ion, therebyresulting in a sharp decrease in the emission intensity of the material.lthas also been determined that replacing terbium, europium, andytterhium with other rare earth ions thatare known .to fluorescein'other environments results in a material that-is eithernonfluorescent or onlyweakly'fiuorescent." For example, the substitutionof a fraction of a percent of cerium results in a material that isnonfluorescent.

Curve 20f the figure depicts the resultsobtained by the instantinvention, yttrium additions toa'potassiumum, lanthanum, andgadoliniumgive results comparable to yttrium in thesodium-yttrium-europium tungstate crystals. Additional measurementsindicate thatterbium and ytterhium rare earth ions giveresultscomparable" to europium in the above tungstate host crystals.

Based on FIG. 2, inclusions of 0.06 to 0.45 gram atoms perforrnula oftrivalent rare earth ion in the ,tungstate host crystal result in amaterial exhibiting an enhanced emission intensity. The lower limit of0.06 gram atoms is based onthe necessity of having sufficient unpairedelectrons available in the negative temperature state toadequately'amplify the input signal. As seen from curve 1, smaller rareearth ion inclusions result in av a maximum and then decrease. The upperlimit of, 0.45"

is further influenced by difficulties encountered in mak-v ing crystalscontaining thehigher doping levels. At doping levels above. 0.45difliculty is encountered in making 0' 7 of tungsticanhydride ,1S .I10t'appreclable at. these tem-.

europium tungstate structure cause no enhancement of emissionintensity.. a l The tungstate crystals of the instant invention are ad'-vantageously grown by the .ditungstate. flux method disclosed in :Patent3,003,112, issued October 3, 1961 to L. G. Van Uitert. Briefly, inaccordance 'with this method, va mixture of. the desired tungstate .anda rare I earth ion-containing composition'is heated in a suitablealkali'metal ditungstate flux to. a temperature sufiicient to form amolten solution. The flux, which is a'solvent for the tungstate and the;rare earth ion-containing composition, :may containan excess of tungsticanhydride in a .molar amount up to the amount of the tungstate presentin the initial mixtureto enhance .the solubility of these components.The molten solution is then slowly cooled until it solidifies. .In' the:course .of cooling, crystals of the tungstate containing .the desiredparamagnetic' ion are formed in the flux.

The initial mixtureis equivalent to 10 mol. parts to mol parts. of thetungstate' and 'moli'parts to 25 mol parts of the flux.- One advantageof the 'flux is. its solvent, power, which permits temperatures 0115900.C. to 1450" C. to be used in forminga molten solution of the mixture.These temperatures avoid reduction of the 'rare' earth ions to lowerundesirable vvalency'stateswhich-are not" suitable for optical maseruse. Additionally, loss peratures, w

Thereis no=critical limitation to particle size of the initialingredients since a moltensolution is formed of the initial mixture;However, it isdesirable to minimize the- 9 amount of accidentally addedrare earth ion impurity in order to insure consistent results. Forexample, the presence of a fraction of a per cent of cerium issuflicient to quench the fluorescence of other rare earth ions in thetungstates. With the exception of cerium, however, accidentally addedrare earth ion impurities are generally tolerated in the compositions ofthe invention in amounts up to 1.0 percent of the principal active rareearth'ion intentionally added. To minimize such conjtamination,spectroscopically pure rare earth substances, such as oxides, aretypically utilized in the initial mixture. Generally, the non-active ionimpurity limits are not'critical and ordinary reagent grade tungstatesor substances that react'to form the tungstates are utilized.

The atmosphere in which the initial mixture is heated is not criticaL-However, it is well known'to use an oxygen-containing atmosphere such asair, oxygen or oxygen plus an inert gas to prevent an ion in a highervalency state such as europium,which is unstable at elevatedtemperatures, from being reduced to a lower valency state. Similarly,for convenience, atmospheric pressure is normally used, althoughpressure is not critical. As is well known, increased pressures ingeneral enhance solubility of the solute, thereby permitting lowertemperatures to be used. After the heating step, the molten solution iscooled at a controlled rate of 01 C. per hour to 25 C. per hour in thesame atmosphere used in the heating step until it solidifies, formingtungstate crystals having fluorescentrare earth ions dispersed therein.The solidification point is readily determined visually. For most of themolten solutions, cooling to a temperature of 650 to 850 C.-is adequateto cause solidification.

The tungstate crystalsin the-flux are then furnace cooled or quenched toroom temperature. The ditungstate flux is removed from the tungstatecrystals by washing the crystals with an alkali such as a solution ofsodium hydroxide. w

Specific examples of procedures utilized in the preparation ofcompositions of the invention are given below. In all cases theproperties of the resulting compositions were measured aspreviouslydescribed and the measuremerits plotted in accordance with thedescription; in

conjunction with FIG. 2. These examples are to be construed asillustrative only and not'as'limiting in any 1 gether. The mixture wasthen heated in a platinum crucible in air for sixteen hours at atemperature of lanthanum tungstate crystals doped withtrivalenteuropiurn. The formed crystals had the following composition:

N o.5 o.4 o.1 4 Example 4 160.3 grams ,Na WO .2H O, 130.3 grams W0 6.10grams La O and 6.60 grams E11 0, were dry mixed. The mixture thenunderwent the same processing as detailed above, with the resultingformation of sodiumlanthanum tungstate crystals doped with trivalenteuropium. The formed crystals had the following composition: V

0.5 o.25 0.25 4 Example 5 160.3 grams Na WO .2H O, 130.3 grams W0 11.95grams Lu O and 2.64 grams Eu O were dry mixed. The mlxture thenunderwent the same processing as detailed above, with the resultingformation of sodium-lutetium tungstate crystals doped with trivalenteuropium. The formed crystals had the following composition:

o.5 0.4 o.1 Q4 Example 6 160.3 grams Na WO .2I-I O, 130.3 grams W0 7.46grams Lu O and 6.60 grams EugO were dry mixed. The mixture thenunderwent the same processing as detailed above, with the resultingformation of sodium-lutetium tungstate crystals doped with trivalenteuropium. The formed crystals had the following composition:

oe oes azs i Example 7 160.3 grams N21 \VO.,.2H O, 130.3 gramsWO 10.9grams Gd O and 2.64 grams B11 0 were dry mixed; The mixture thenunderwent the same processing as detailed above, with the resultingformation of sodium-gadolinium tungstate crystals doped with trivalenteuropium. The formed crystals had the following composition:

oj on oaw Example 8 mixture then underwent the'same processingasdetailed 1100 ,C. The molten solution formed was then cooled in air at acontrolled rate of 2.5 C. per hour to a temperature of 700 C. Theresulting solids-were then furnace cooled to room temperature and washedwith hot sodium hydroxide, leaving sodium-yttrium tungstate crystalsdoped with trivalent europium." The formed crystals had the followingcomposition:

e.5 o.4 o.1 4

Example 2 Example 3 160.3 grams N21 WO 2H O, 130.3 grams W0 9.76 gramsLa 'O and 2.64 grams Eu O were dry mixed.

The mixture then underwent the same processing as detailed above, withthe resulting formation of sodiumabove, with-the resulting formation ofsodium-gadolinium tungstate crystals doped with trivalent europium. Theformed crystals had the following composition:

o.5 o.25E o.25 4 Examp e 9 160.3 grams Na WO 2I-I O, 130.3 grams W0 6.78grams 03 and 2.74 grams Tb O were dry mixed. The mixture then underwentthe same processing as detailed above, with the resulting formation ofsodium-yttrium tungstate crystals doped with trivalent terbium. Theformed crystals had the following composition:

Example 10 160.3 grams Na WG 2H O, 130.3 grams W0 9.76 grams 1321 0 and2.74 grams Tb O were dry mixed. The mixture then underwent the sameprocessing as detailed above, with the resulting formation ofsodium-lanthanum tungstate crystals doped with trivalent terbium. Theformed crystals had the following composition:

o.5 o.4T 0.1 4 Example 11 160.3 grams Na WO ZI-I O, 130.3 grams W0 11.9grams Le o, and 2.74 grams Tb O were dry mixed. The mixture thenunderwent the same processing as detailed formed crystals had thefollowing composition:

- o. uo.4T u.1W r

Example 12 @603 grams .Na wo .2,rr 0,-130.3 grams W03, 10.9 7

grams Gd O and 2.74 grams Tb Ogwere dry mixed. The mixture thenunderwent the same processing as detailed above, with the resultingformation of sodium-gadolinium tungstate crystals doped with trivalentterbium. The formedcrystalshad the following composition:

tlLEG OAT jWOQ Example 13 p .160.3.grams Na WO .2H O,'130.3 grams WO','6 73 grams Y O and 2.96 grams Yb O were dry mixed. The mixture thenunderwent .the same processing as detailed above, withtheresultingformationzof sodium-yttrium tun'gstate crystals doped withtrivalent ytterbium. The

formed crystalshad the following composition:

oeL ois oaw r Example 15 1 60.3 grams Na WQ ZI-I O, 130.3 grams W0 11.95

above, with the resulting formation of sodium-lutetium 'tungstatecrystals' doped with trivalent terbium.v The grams D1 0 and 2.96grams:Yb O were dry mixed. The

I mixture .then underwent the same processing as detailed above, withthe resulting formation of sodium-lutetium tungstate crystalsdoped withtrivalent ytterbium. The formed crystals. had thevfollowing composition:

0.5 e.4Y o.1W 4 Example '16 160.3 grams Na Wo..2H,;0, 1303 gramsWO3,10.9

wherein B iseuropium; I

grams Gd O and 2.96 grams Yb o iw'ere'dry mixed; The i mixture thenunderwent the same processing as detailed above, with the resultingformationof sodium-gadolinium tungstate crystalsdoped with trivalentytterbium. The formed crystals had the following composition:-

' tis bAY oJ Example 17 The crystalsformed in accordance with .theprocedure the. end of a platinum wire and the tip of the crystal wasthen immersed "in the above melt. The crystal was ro- 2,929,922 3/60*Schawlow 88 -61 tated at Tr.p.m. and'slowlypulledout of the melt at arate. of one inch per hour.: At the end of one hour a rod-shaped crystalone inch long was'formed. The crystal :had. approximately the samecomposition as the original seed. crystal on which it was. formed,

Example 18 j The elongated crystal formed in accordance with thesame:composition as the original crystal on which it was formed."

Whatis claimed. is: I

1. A i composition. 'ofmatter consisting essentially of. a singlecrystal, tungstate materiallhaving a crystalline lattice. of:thescheelitestructure and an empirical formula Na A B WO where A'is anion selectedfrom the group consisting of V lutetium, yttrium, lanthanum,and gadolinium, B is-a trivalent.rare earth ,ionselected from the groupconsisting of europium, terbium, and ytterbium, and x has a value offrom 0.06 to 0.45.

2. A composition of matter in accordance with claim 1 wherein x hasavalue of from. about 0.08. to about 0.4.

3; A composition of matter in accordance with claim 1 wherein x has avalue of vfronrabout 0.10 to about 0.35. i

4. A composition of matter. in accordance with clairnl wherein A isyttrium.

5. A compositionjot matter inaccordance with claim 4 wherein B iseuropium. I

6. A compositionof matter in accordance with claim 1 wherein A islanthanum.

7; A composition of-matter in accordance Withclaim 6 wherein B iseuropium. V

.8. A compositionof matter in accordance with claim '1 wherein A islutetium.

9. A composition of matter in accordance with claim 8 10. A compositionof matter in accordance with claim 1 wherein A is gadolinium.

11. A "composition of matter'in accordance with claim 10 wherein B iseuropium. i

References Cited by the Examiner" UNITED STATES PATENTS 1 OTHERREFERENCES Kroger: Some Aspects of; the Luminescence .of Solids,Elsevier-Pub.xCo., New York, 1948 pages 109, 286, 290,

- 291, 293, 297, and'298. g

MAURICE A. BRINDISI, Primary Emittin JOSEPH R. LIBERMAN, Examiner.

1. A COMPOSITION OF MATTER CONSISTING ESSENTIALLY OF A SINGLE CRYSTALTUNGSTATE MATERIAL HAVING A CRYSTALLINE LATTICE OF THE SCHEELITESTRUCTURE AND AN EMPIRICAL FORMULA NA0.5A0.5-XBXWO4, WHERE A IS ANIONSELECTED FROM THE GROUP CONSISTING OF LUTETIUM, YTTRIUM, LANTHANUM, ANDGADOLINIUM, B IS A TRIVALENT RATE EARTH ION SELECTED FROM THE GROUPCONSISTING OF EUROPIUM, TERBIUM, AND YTTERBIUM, AND X HAS AVALUE OF FROM0.06 TO 0.45.